The invention concerns a measuring apparatus or probe for continous determination of oxygen partial pressure in liquid and in gases particularly in blood. The measurement of oxygen partial pressure is carried out with the aid of gas-sensitive electrodes and in particular by the determination of the reduction current which flows as a result of the attachment of the oxygen to the cathode as soon as a potential source is applied between the cathode and the anode and the two electrodes are in communication with each other via an electrolyte. Although the course of the reaction is still not fully understood, one may assume that the oxygen which is present at the electrode is reduced there by the attachment thereto of electrons and, in accordance with the following equation, is converted into hydroxyl ions. EQU O.sub.2 +2H.sub.2 O+2e.sup.- .revreaction.H.sub.2 O.sub.2 +2OH.sup.- EQU H.sub.2 O.sub.2 +2e.sup.- .revreaction.2OH.sup.-
At the anode a portion of the atoms of the anode material is oxidized to ions which is equivalent to the electron surrender at the cathode, thus in the case of the silver anode, silver atoms are converted to silver ions (Ag.sup.+). The thus flowing current is dependent upon the number of oxygen molecules which arrive at the cathodic surface. Where the potential difference between the anode and the cathode is sufficiently large to reduce all oxygen molecules which arrive at the cathode, then in the inner portion of the plateau of the polarogram even a raising of the potential will not bring about an increase in the current. That is to say that the current flow is then independent of the applied potential and is only determined by the number of oxygen atoms which arrive at the cathode. This current provides a quantitative standard for the determination of the oxygen partial pressure present in the test solution.
In order to carry out the measurement of the oxygen partial pressure heretofore it has been preferred to use measuring arrangements with a glass enveloped cathode made of platinum or other noble metals which is separated from the measuring chamber, in which the test sample is to be found, by a membrane which is impermeable to ions but is permeable to oxygen. The measuring probe itself for example, comprises a glass enveloped platinum electrode, whose tip is freely accessible in the end surface of the glass envelope. Silver electrodes are the most frequently used as the anode which, in order to increase the stability of the readings therefrom, may be coated with a layer of silver chloride. Anodes and cathode tips are surrounded and connected by a solution of electrolytes, suitably a 0.2 molar solution of potassium chloride.
The aforementioned separation of the test material from the measuring sensor has the purpose of preventing changes in the electrodes due to precipitation, for example by reducing or oxygenating substances present in the test material. The exactness of the measurement is, however, also influenced by an observed instability of the measuring probe itself. Research in this area has shown that the determination of the oxygen partial pressure by measurement of the current flow between a cathode and an anode is falsified by a superimposed current which, to a substantial extent, occurs as the result of the migration of ions of the anode material, for example the silver ions to the cathode. This so called silver ion drift also occurs when the anode is covered with a silver chloride layer since experience has shown that such a coating contains certain gaps through which the silver ions may pass. As one has seen through microscopic examination of the free cathodic surface, it has not been possible to prevent the contamination of the free cathodic surface by the migration of silver ions, such contamination becomes worse with time of use of the device. These changes in the free cathodic surface are the principle cause of the measuring instability of the measuring probe and a source of error in the determination of oxygen partial pressure.
It would be desirable therefore, to provide a measuring device for the continuous determination of oxygen partial pressure in fluids and gases, in particular in blood, whose measuring value stability vis a vis the known measuring devices is improved so that the measuring arrangement may be used for a longer operating period of time without the need for interrupting the measurement to clean the cathode.